This application relates to the art of leaching residual solvents from polybenzazole films. For the purposes of this application. the term "film" refers to both thin film (usually at most about 10 to 15 mil thick) and to sheet (usually more than about 10 to 15 mil thick).
It is well-known to polymerize polybenzazole polymers in acid solutions, so that an acid dope is formed. See. e.g., Wolfe et al., U.S. Pat. No. 4,533,693 (Aug. 5, 1985) which is incorporated herein by reference. Frequently, the resulting polymers are not thermoplastic. However, they can made into films by: (a) extruding the desired film out of the dope; (b) optionally stretching the dope; (c) coagulating the polymer from the dope; and (d) leaching the residual solvent (usually phosphoric acid) out of the film. See, e.g., Harvey et al., U.S. Pat. No. 4,973.442 (Nov. 27, 1990) which is incorporated herein by reference.
The leaching step can become a significant bottleneck in a continuous production process, because it is very time consuming. Commercially desirable films should contain no more than about 2000-2500 ppm phosphorus, and film for very sensitive uses should contain about 250 ppm phosphorus or less. On the other hand, the Harvey patent shows that film washed in water for 5 minutes contains 4 weight percent phosphorus, and film washed for 48 hours still contains 0.8 weight percent phosphorus (8000 ppm). The long leaching times needed to reach acceptable levels of phosphorus are unmanageable in a commercial production.
What is needed is an accelerated process to leach residual solvent from polybenzazole films.